The present invention relates generally to the rotor of a superconducting generator and more specifically to the cooling loop whose function is to provide helium flow in contact with the generator rotor's copper electromagnetic cold shield.
Typically, a superconducting generator utilizes a supercooled rotor which includes a superconducting field winding. A fluid refrigerant, for example liquid helium, is used to supercool the winding to a cryogenic temperature. Replacing the conventional, copper conductor field winding in the rotor of a synchronous generator with a high capacity superconducting winding that has virtually zero resistance at cryogenic temperatures results in some important benefits. The most obvious benefit is the elimination of rotor I.sup.2 R loss. Rotor ventilation power requirement reductions accompany the resulting reduction in excitation power. More subtle, but nonetheless beneficial, attributes of the superconducting synchronous generator are the increased power density and the elimination of stator iron in the armature winding.
Also, since the airgap armature winding can be electrically isolated from ground potential, the generator winding can be developed to operate at electrical voltages equal to transmission line voltages, thus eliminating the need for generator step-up transformers. Finally, the stronger magnetic coupling between the rotor and stator of a superconducting generator's magnetic circuits can make a major contribution to electrical system static and dynamic stability performance.
One criterion for stability is that superconducting generators for power plant application must be designed to remain in the superconducting state through the power system's most severe operating condition: the three-phase, high-voltage transmission line fault interrupted by a backup breaker after 15 cycles (250 msec). When a transmission line fault occurs, the rotor's superconducting windings are subjected to heating caused by currents that are induced to flow in the cold shield by time-varying magnetic fields. This heating can readily cause a transition to the resistive state if the windings are not shielded from it. Resistive transitions (quenches) result in a generator outage. In order to be judged fault worthy, the superconducting rotor must be able to withstand these transmission line faults without having its field windings rise above superconducting temperatures and into the resistive state.
Faults in the power system (in the high voltage side of the generator's step-up transformer) cause a sudden increase in the load on the generator resulting in a decrease in its speed of rotation. This is quickly followed by an increase in turbine power output as it attempts to maintain synchronization with the transmission line frequency. Soon afterward (3-6 cycles) circuit breakers open to remove all of the load from the generator, greatly reducing its power consumption. This, coupled with the turbine's attempt to increase its power output, results in an instantaneous overspeed condition. As the turbine continues to attempt to maintain synchronization with the transmission line frequency its speed oscillates about this frequency, first above and then below the speed required for synchronization. This "hunting" creates a time-varying magnetic field which penetrates the superconducting rotor.
On the outside of the rotor is a damping shield, at room temperature, in which currents flow due to these magnetic fields. This heating dissipates energy thereby damping the oscillation of the rotor. This action causes the damping shield to compress and perform work in attempting to distort the magnetic field inside the warm damper. To prevent a subsequent heating of the superconducting rotor windings, a cold shield is placed radially outside of the windings and inside of the damping shield. The function of this copper shield is to remain rigid and allow currents to flow in it that oppose the magnetic field variation generated from the damper shield's motion. The hunting of the generator lasts for a few seconds during which the above-mentioned energy dissipation and resulting current flow exist.
Radially inside of the cold shield and outside of the rotor's field windings is a stainless steel cylindrical field winding retaining tube. The stainless steel cylinder has a very low thermal diffusivity and therefore will act as a heat barrier in delaying the progress of the resulting heat wave from the cold shield radially inward to the rotor windings. However, if the cold shield is not cooled before the heat wave reaches the winding, the winding temperature will rise above superconducting temperatures and a transition to the resistive state will result. The present invention deals directly with this problem and the requirement that an adequate coolant flow exists immediately after the cold shield begins to heat in order that the superconducting generator be fault worthy.
During normal operation of the supercooled rotor the cold shield prevents externally radiated heat from reaching the windings. The heat is removed from the cold shield by a cooling loop which passes a slow flow of coolant from the helium pool radially outward through a passage in a radial heat exchanger and into a circumferential channel from which the coolant then flows axially through multiple axial channels, into another circumferential channel and then radially inward back into the helium pool. The radial heat exchanger maintains the inlet end of this cooling loop at a lower temperature than the outlet end. This temperature difference and its resulting differential coolant densities create a thermosyphon which maintains the slow flow of coolant required to remove the radiant heat from the cold shield during normal operating conditions.
However, during the occurrence of a transmission line fault and the resultant time-varying magnetic fields as described above, the helium in this cooling loop rapidly expands. This expansion would normally cause helium to flow in both directions away from the middle of the loop which is adjacent to the cold shield. Although this two-directional flow lasts for a very short time it would be sufficient to destroy the heretofore steady slow flow of helium through the cooling loop. Following the sudden expansion of helium and destruction of steady coolant flow the helium, if given sufficient time, would again set up a proper cooling loop flow when the temperature differential between the inlet and outlet ends of the loop is reestablished. However, this required time is greater than that afforded by the low thermal diffusivity of the stainless steel field winding retaining tube. By the time that a cooling loop coolant flow is reestablished the rotor's field winding would pass from the superconducting to the resistive state.
To prevent this transition to the resistive state, a means is required for rapidly restarting the thermosyphon in the cooling loop following a thermal transient. This thermosyphon must be restarted within the period of time that it takes for the heat wave to pass through the stainless steel field winding retaining tube and reach the rotor's superconducting field winding.